The present invention relates to a back light source for liquid crystal display device used for personal computers and car navigation systems, and particularly to a lighting device for dielectric barrier discharge lamp suited for the back light source.
Conventionally, cold cathode fluorescent lamps enclosed with mercury have been used for back light sources for liquid crystal display device. FIG. 1 shows a waveform for driving a lamp, i.e. a lamp voltage waveform and a current waveform of a cold cathode fluorescent lamp enclosed with mercury operated under a low dimming control ratio (1%). In case of the fluorescent lamp enclosed with mercury, the waveforms of the lamp voltage and the current are generated in a period W corresponding to the dimming control ratio of 1%, using signals of high level (H) period of dimming control signal LCS corresponding to the dimming control ratio of 1% as shown in FIG. 1. In case of the fluorescent lamps enclosed with mercury, even if initial waveform distortion or residual waveform existed a little in the vicinity of the period W, the lamp light did not show flickering. This is because mercury is lighter than xenon in weight and it can move freely inside the glass tube in wide range. As a result, if a lamp voltage having a little distorted waveform is supplied to a fluorescent lamp enclosed with mercury, a phenomenon of the flickering of the lamp hardly took place because ultraviolet ray emitted from mercury atom diffuses widely.
Because mercury has a very high conversion efficiency of light compared with rare gas such as xenon, there is an advantage that enclosing a minute amount of mercury in the glass tube is sufficient.
However, in recent years, a dielectric barrier discharge type cold cathode fluorescent lamp using rare gas with no mercury contained, which is a hazardous substance, has been developed. One example of the lighting device for lighting such rare gas fluorescent lamps is explained using FIG. 2 to FIG. 8.
FIG. 2 and FIG. 3 are a side view and a longitudinal sectional view respectively showing a construction of an external electrode type fluorescent lamp which is a type of dielectric barrier discharge lamp. The external electrode type fluorescent lamp 11 is provided with a glass tube 1, inside which a discharge medium containing at least xenon is enclosed, and on an inner wall of which a phosphor 2 is provided. On at least one end of the glass tube 1, an inner electrode 4 is sealed with a lead-in wire 3 being connected. Around an outer surface of the glass tube 1, an electrically conductive wire is wound spirally as an external electrode 5. The external electrode 5 is coated with a translucent heat shrinkage tube 6 and is fixed on the outer surface of the glass tube 1. The inner electrode 4 is connected with a power supply wire 8 through a lead-in wire 3, and the external electrode 5 is connected with a power supply wire 8′ through a fixing metal bar 7. The external electrode type fluorescent lamp 11 starts discharging, when it is supplied high frequency pulses alternating between positive and negative voltages, which is applied between the inner electrode 4 and the external electrode 5 from an HF power source device 9 through power supply wires 8, 8′. The HF power source device 9 is composed of inverters, for example. Thus the external electrode type fluorescent lamp 11 emits an ultraviolet ray from xenon. The ultraviolet ray is then converted into a visible light by phosphor 2 and is used as a light source.
FIG. 4 and FIG. 5 are block diagrams showing a circuit configuration of the HF power source device 9 for lighting the external electrode type fluorescent lamp 11. The HF power source device 9 includes an inverter circuit 10, which converts a DC voltage into an AC pulse signal. In the inverter circuit 10, the external electrode type fluorescent lamp 11 is connected with the secondary coil of the transformer T1. Capacitors C1 and C2 for providing center bias are connected with one end of the primary coil. The capacitors C1 and C2 are connected in series between a DC source Vcc and a ground (GND), and the connecting point of the capacitors C1 and C2 is connected with the end of the primary coil of the transformer T1. Semiconductor switching elements S1 AND S2 and circuit elements Z1, Z2 are connected in series also between the DC source Vcc and the GND. Another end of the primary coil of the transformer T1 is connected with the connecting point of circuit elements Z1 and Z2. The circuit elements Z1 and Z2 are composed of coils, diodes, and some other elements having resistance components such as resistors or group of elements formed by combining them.
The semiconductor switching elements S1 and S2 are supplied with driving signals P1 and P2 produced by a drive signal generating circuit 12 through dimming control circuit 13, thereby performing respectively the switching control by these driving signals. The drive signals P1 and P2 have phases different from each other by 180° and have an equal repetition cycle t. The dimming control circuit 13 is a gate circuit which controls the number of drive signals P1 and P2 passing through it in a time period T defined by the dimming control signal LCS. When drive signals P1 and P2 are supplied to semiconductor switching elements S1 and S2, the semiconductor switching element S1 is turned OFF and the semiconductor switching element S2 is turned ON. Then, positive lamp current IA flows along a pass shown by a dotted line in FIG. 4. In the next cycle of the drive signals P1 and P2, the semiconductor switch S1 is turned OFF and the semiconductor switch S2 is turned ON. Then, negative lamp current IB flows along a pass shown by a dotted line in FIG. 5.
That is, when the voltage at the primary coil of the lamp drive transformer T1 is changed in a manner as L→H→L→H→L→H . . . , in accordance with the ON period of the drive signal P1, and drive signal P2, positive and negative lamp current are supplied to the external electrode type fluorescent lamp 11 connected with the secondary coil of the transformer T1, thereby lighting the fluorescent lamp 11 as shown in the timing chart of FIG. 6 and FIG. 7. By repeating the operations, a square wave voltage is supplied to the lamp 11 continuously. As a result, positive and negative lamp currents are applied continuously to the external electrode type fluorescent lamp 11 using the rare gas, thereby achieving the lighting of the lamp with high luminance.
Here, generally, the lighting device is provided with a dimming control device which controls the luminance of the lamp according with the surrounding area. The luminance of a lamp adjusted by the dimming control device is indicated by a dimming control ratio. The dimming control ratio is indicated by an arbitrary luminance to the maximum luminance of the lamp in %.
The dimming control signal 14 shown in FIG. 4 and FIG. 5 is a so called pulse width modulation signal in which a pulse width is changed in according with the dimming control ratio. In control circuit 13, switches S3 and S4 are closed only in H (high) voltage period of the dimming control signal 14 and supplies drive signals P1 and P2 generated in the drive signal generating circuit 11 to the semiconductor switching elements S1 and S2. The switches S3 and S4 are opened in the L (low) voltage period and the drive signals P1 and P2 are not supplied to semiconductor switching elements S1 and S2. Here, the period of the high voltage (H) of the dimming control signal becomes maximum when the dimming control ratio is 100%, and it becomes narrow in accordance with the dimming control ratio. As a result, a different numbers of the drive pulses in a fixed period are provided as the drive pulse signals P1 and P2 from the control circuit 13 in accordance with the dimming control ratio.
Here, the conventional HF source device 9 is difficult to use in circumstance where a stable lamp operation is required, since the flickering is prominent when the barrier discharge lamp using a rare gas without containing mercury operated at a low dimming control ratio of 25% or lower. The present inventors investigated the cause of the flickering and finally found that distortions in the waveform or variation in repetition cycle of the drive signal P1 and P2 cause the phenomenon, which will be explained bellow.
FIGS. 6–8 are timing charts showing the waveforms of the lamp drive signal P1 and P2 in HF source device 9. In the figures, the timing chart shows the waveforms when the repetition frequency of the lamp drive signal P1 and P2 is 20 kHz respectively and the dimming control ratio is 2%. Here, the fact that the lamp drive frequency (=f) is 20 kHz means that the repetition cycle t of the drive signal P1 and P2 is 50 μs. Now, if an unit time is chosen as 10 ms (where the repetition cycle is 100 Hz), numbers of pulse per unit time is 200. Hereinafter, the unit time is called as the dimming control signal cycle or terminal T. That is, when the dimming control ratio is 100%, 200 pulses per 1 cycle T of the dimming control signal are repeatedly supplied to the lamp 12 at 100 Hz frequency as the drive pulse P1, P2. Therefore, when the dimming control ratio is 2.0%, 4 pulses per 1 cycle of the dimming control signal are supplied to lamp 12 at 100 Hz frequency. The drive signal P1 and P2 have phases different from each other by 180°. Thus the drive signal P1 is in H level when the drive signal P2 is in L level, and on the contrary the drive signal P1 is in L level when the drive signal P2 is in H level. However, in the waveform of the drive signal P2 shown in FIG. 6, a lack portion A-1 is generated in the first pulse waveform of the drive signal P2 in the first period T(1) of the dimming control signal. As a result, a lack portion A-2 is generated in the lamp current waveform IL1 shown in FIG. 6. Further, in the second period T(2) of the dimming control signal, a lacking portion B-1 at the last pulse waveform of the waveform of the drive signal P1 is generated, and a lacking portion B-2 is generated at the portion corresponding to the lamp current waveform IL1. Here, in this case, lacking portions A-3, B-3 are generated at the corresponding portion of the waveform of the primary coil voltage V1 of the lamp drive transformer T1.
In the timing chart of FIG. 7, it is shown that the waveforms of lamp drive signal P1 and P2 start earlier by a time period of (t+α) compared with the period t of 1 cycle of the lamp drive signal P1 and P2 in the second period of the light dimming control signal. Here, the variation α of the start timing in the second period surpasses a range of −½×t˜+½×t. As a result, both the waveforms of the voltage V1 in the primary coil of lamp drive transformer T1 and of the lamp current IL1 start earlier by the period of (t+α) in the second period of the dimming control signal.
FIG. 8 shows a timing chart, in which the phases of lamp drive signal P1 and P2 waveform themselves vary at each repetition cycle t and the variation exceeded the range of − 1/10×t˜+ 1/10×t. As a result, the primary coil voltage V1 of lamp drive transformer T1 and lamp current waveform IL1 vary at each repetition cycle t respectively, the variation exceeds the range of − 1/10×t˜+ 1/10×t.
It becomes clear that the semiconductor switching elements S1 and S2 are ON/OFF controlled by the drive signals P1 and P2 containing such waveform distortion or cycle variation of dimming control signal. As a result, the similar waveform distortion or the variation in each cycle arises in the drive voltage waveform V1 of the external electrode type fluorescent lamp 11 connected with the secondary coil of the transformer T1, thereby occurring the flickering in the luminance of lamp 11.
On the contrary, the flickering of the lamp did not occur because the light emitting principle differs from that of the external electrode type fluorescent using the rare gas and free from mercury, when the fluorescent lamp containing mercury in the discharge gases is operated by the conventional HF source device 9, even if the distortion such as lack in the lamp drive waveform or fluctuation in the cycle of dimming control signal existed in such a low dimming region as 25% or lower.
The present invention is made to solve the technical problems in the conventional lighting device mentioned above. Therefore, one of the objects of the present invention is to provide a lighting device for dielectric barrier discharge lamps, which enables a stable lamp operation without flickering in lamp luminance at a low dimming ratio, such as of 25% or lower.